1. Field of the Invention
The present invention relates to a transfer type color image forming apparatus, e.g., a color laser printer, a color digital copying machine, or the like and, more particularly, to a driving technique of an image carrier and a transfer medium support member of the color image forming apparatus.
2. Description of the Related Art
For example, in a transfer type color image forming apparatus such as a color laser printer, an image carrier such as a photosensitive drum and a transfer medium support member such as a transfer drum are brought into contact with each other, and are synchronously rotated. Beam light is scanned on the image carrier to form a latent image on the image carrier, and the formed latent image is developed into a color toner image. The developed color toner images are sequentially transferred onto a transfer medium supported and conveyed by the transfer medium support member to overlap each other, and the color toner images on the transfer medium are thermally fixed.
In such a color image forming apparatus, conventionally, a motor controller using Phase-Locked Loop (PLL) control is used in synchronous rotation between the image carrier such as the photosensitive drum and the transfer medium support member such as the transfer drum.
When a phase comparison output as a result of phase comparison in the PLL control is filtered through a low-pass filter, a phase control signal having a signal level proportional to a phase difference between a phase comparison reference signal and a Frequency Generator (FG) signal is supplied within a phase difference range between -.pi. and +.pi..
However, when an abnormality such as an abrupt variation in load for a motor occurs, no FG signal at the fifth cycle may be input at the fifth cycle of a phase comparison reference signal, and the FG signal at the fifth cycle may be input at the sixth cycle, as shown in, e.g., FIG. 8B.
In this state, the phase comparison reference signal and the FG signal at the fifth cycle should have a phase difference of +.pi. or more. However, the phase comparison reference signal at the sixth cycle and the FG signal at the fifth cycle are compared with each other, and a phase comparison result is output in a state wherein a phase is shifted by 2.pi.. More specifically, as shown in FIG. 8A, the same output as in a phase range between -.pi. and +.pi. is undesirably obtained in a range shifted by an integer multiple of 2.pi. from a given phase difference.
As shown in FIG. 8C, assume that a variation in speed begins to occur at a point A in FIG. 8C, the speed is decreased up to a point B, and the phase difference between the FG signal of a motor and the phase comparison reference signal exceeds a range of .+-..pi.. Thus, the motor is PLL-controlled in a phase (point C) delayed by an integer multiple of 2.pi. from the phase comparison reference signal, and the phase delay (corresponding to a color misregistration amount in color image formation) cannot be recovered.
When the above-mentioned motor controller based on the PLL control using a phase comparator is applied to synchronous rotation between the image carrier and the transfer medium support member in the transfer type color image forming apparatus, even when image formation of the first color is ideally performed, if the phase difference between the FG signal of a motor and the phase comparison reference signal exceeds a range of .+-..pi. in the second color due to any variation in load, the motor is undesirably controlled in a phase advanced (delayed) by an integer multiple of 2.pi. from the phase comparison reference signal, in other words, in a state wherein the rotational position is shifted, and image formation continues without recovering the phase advance (delay). Color misregistration in the sub scanning direction occurs in the output image after a portion causing a variation in speed.